A polyamide has excellent mechanical properties and processability and has relatively high gas-barrier performance, and is therefore widely used not only for injection-molding materials for automobile parts, electric-electronic parts, and the like but also for packaging materials for foods, drinks, chemicals, electronic parts, and the like as well as for industrial materials. For example, a polyamide to be obtained through a polycondensation reaction of a xylylenediamine and an aliphatic dicarboxylic acid has high strength and a high elastic modulus and has low permeability for gaseous substances, such as oxygen, carbon dioxide gas, odor, and flavor, and is therefore widely used for gas-barrier materials in the field of packing materials. In addition, a polyamide has resistance and gas-barrier performance against alcohols, weakly-alkaline chemicals, weakly-acidic chemicals, fuels, various types of organic solvents, industrial gas, and the like, and is therefore also widely used for materials in the field of industrial application.
Various methods for production of polyamide resins are known. For example, there is known a method involving: obtaining a polyamide through polycondensation of a dicarboxylic acid component and a diamine component; and then increasing the degree of polymerization of the polyamide by a solid-phase polymerization method using a heating and drying apparatus, such as a tumble drier. However, the solid-phase polymerization method is limitative in producing a resin having a high melting point or in increasing the molecular weight of a resin having low crystallinity, and is therefore problematic, for example, in that the method cannot provide a resin having a high melting point.
Meanwhile, there is also known a method of obtaining a polyamide resin by melt-kneading a polyamide oligomer which has been obtained through polycondensation of a dicarboxylic acid component and a diamine component using a twin screw extruder for further polycondensation of the polyamide oligomer (for example, see PTL 1). The advantages of the twin screw extruder are that an oligomer can be melt-kneaded therein within a short period of time, a polyamide resin having a high melting point can be produced, and many kinds of products in small amounts can be processed by virtue of its self-cleaning performance. Accordingly, investigations have been made on various methods of producing various types of polyamide resins using the twin screw extruder.